Rogue Genes South of the Border

While preparing a workshop on DNA detection in Oaxaca, Mexico, David Quist found a surprise: an alien gene embedded in a sample of native corn, known as criollo. The alien was actually familiar, a type of gene commonly found in the genetically modified crops grown in the United States. Indeed, Quist observed the same DNA signature in a can of American corn he had brought for comparison. But this was an ancient strain of maize cultivated in the remote mountains of southern Mexico. How did the gene get there?

Quist and Ignacio Chapela, both microbial ecologists at the University of California at Berkeley, performed a comprehensive study of criollo in the region. Sampling four fields more than 12 miles from the closest mountain road, the researchers discovered that the native corn had incorporated not one but several genes found in engineered American corn. One sample of criollo even contained the gene for Bt toxin, an insecticide derived from bacteria.

How these genes migrated into Mexican criollo remains a mystery. Pollen grains can carry genes from engineered plants into nearby native strains or even into closely related weeds, but in 1998 Mexico banned the planting of genetically modified crops. Chapela suspects farmers may be planting imported corn originally distributed as food, or that migrant workers may be bringing samples back from the United States. Whatever its source, the genetic pollution could have grave consequences. Corn originated in this region, and it is here that it is most genetically diverse. "If you lose that diversity, you lose the possibility of finding disease-resistance genes in the future," says Chapela. "It's a serious challenge to food security worldwide."